The success of the adaptive immune system depends upon the rapid generation of effector cells and the maintenance of memory to the antigens and pathogens previously encountered. The germinal center (GC) is the site of somatic hypermutation of antibodies, antibody class switching to IgG, and a central source of memory B cells and thus is critical for effective humoral immunity. The formation of GCs within secondary lymphoid organs after challenge with protein immunizations or viral infections is driven by specialized CD4+ T follicular helper (TFH) cells that interact with antigen-specific B cells to initiate the GC. GC-dependent neutralizing antibodies (Abs) that develop are critical for viral control and defense against secondary infection and are utilized as biomarkers for the efficacy of influenza vaccines. Thus, there is a significant need to understand the pathways that lead to differentiation of TFH, GCs, and memory B cells. The cognate interactions between MHC class II positive antigen presenting cells (APC) and antigen-specific CD4+ T cells required for TFH differentiation, the GC reaction, and memory B cell development in different settings and sites remain poorly described. To address this question, we developed a series of mice with altered expression of MHC class II on dendritic cells (DCs) and B cells and examined the immune response to model protein antigens and viral infection. Our preliminary results show that: 1. TFH differentiation is a multi-step process in which conventional DCs are critical for initial priming events but B cells are necessary for imparting the epigenetic changes that mediate complete effector potential; 2. Altering the timing of secondary B cell-T cell interactions preserves the differentiation of TFH bu inhibits the GC response; 3. In contrast to current models, T cell- dependent memory B cell differentiation can be preserved in the absence of robust GC responses; 4. APC requirements for TFH differentiation are relaxed following viral infection. These results lead to our hypothesis that variable spatial and temporal control of TFH development, the germinal center reaction, and B cell memory formation can be manipulated to impact the response to viral infection or immunization. In the current proposal, we will utilize a number of murine models to dissect the APC-T cell interactions that regulate TFH differentiation and the GC response following influenza infection. First, we will define the MHCII- dependent cognate interactions that regulate clearance and protection from influenza A. Mice with limited expression of MHCII will be vaccinated against or infected with Influenza A. We will determine which APCs process and present individual T cell epitopes recognized by effector T cells and TFH and when TFH differentiation is sufficient to drive GC responses and the development of B cell memory. We hypothesize that DCs initiate TFH differentiation but secondary Ag presentation from B cells is necessary to imprint the lineage. To advance this model, we will examine genome wide histone modifications of the TFH lineage induced by different APCs. These data will generate an epigenetic and transcriptional roadmap of the humoral response and define how individual APCs synergize to fix the functional lineages. Finally, we will utilize our models to determine if pharmacologic manipulation with improved adjuvant or epigenetic modification enhances vaccination. Completion of these Aims will provide significant insight into the biology of TFH differentiation and differentiation of the germinal center and B cell memory responses during influenza vaccination and infection. We will identify cellular and molecular targets that regulate humoral immunity and protection to guide future investigations to enhance the immune response to a virus that is problematic for Veterans.